Variable Temperature Capacitance-Voltage Measurements to Investigate the Density of Localized Trapping Levels in Organic
- PDF / 113,138 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 26 Downloads / 197 Views
0905-DD06-02.1
Variable Temperature Capacitance-Voltage Measurements to Investigate the Density of Localized Trapping Levels in Organic Semiconductors
N. Sedghi1, D. Donaghy1, M. Raja1, S. Badriya2, S. J. Higgins2, and W. Eccleston1 1. Department of Electrical Engineering and Electronics, The University of Liverpool, Brownlow Hill, Liverpool L69 3GJ, UK. 2. Department of Chemistry, The University of Liverpool, Crown Street, Liverpool L69 7ZD, UK.
ABSTRACT We have approximated the tail of the Gaussian distribution of states of organic semiconductors with an exponential function. We have used this approach to calculate the carrier concentration in organic materials, and subsequently the charge distribution in the accumulation region of a field effect device and the space-charge capacitance in accumulation mode. Small signal high frequency capacitance-voltage measurements performed at various temperatures show good agreement with this model and the characteristic temperature of the exponential function has been estimated from these measurements based on the theory developed. INTRODUCTION Charge transport in organic disordered semiconductors occurs via variable range hopping (VRH) among localized states [1-2]. The density of states (DOS) in organic materials is commonly believed to be described by a Gaussian distribution of energy sites [3]. Study of the density of states distribution is crucial to the understanding of conduction mechanism in disordered organic semiconductors. The tail of a Gaussian distribution can be approximated to an exponential function to make the analytical calculations more practical. A key parameter in an exponential function is the characteristic temperature, TC, which is an important parameter in modeling organic thin-film transistors (OTFT) and other organic devices. The width of Gaussian function can also be related to this parameter. The exponent parameter in the universal power law [4-5] also depends on the value of TC [6]. We have used temperature dependent high frequency capacitance-voltage (CV) measurements to estimate the characteristic temperature TC. THEORY The density of states (DOS) in organic materials is commonly described by a Gaussian distribution of energy sites g (E ) =
⎛ E2 ⎞ NT ⎟ exp⎜⎜ − 2 ⎟ 2π σ ⎝ 2σ ⎠
(1)
where NT is the total concentration of states, σ is the standard deviation (or Gaussian width), and E is the energy. The tail of the Gaussian function, for any particular energy range, can be approximated to an exponential function [7] (see Figure 1)
Density of States, N(E)
Density of States, N(E)
0905-DD06-02.2
Energy, E
Energy, E
(a)
(b)
Figure 1. (a) Linear and (b) logarithmic plots of Gaussian (dashed line) and exponential (solid line) densities of states. g (E ) =
⎛ E − Ei Ni exp⎜⎜ kTC ⎝ kTC
⎞ ⎟⎟ ⎠
(2)
where Ni /kTC is the density of states at energy level Ei, k is the Boltzmann constant, and TC is a characteristic temperature. This approximation applies to most situations, since the majority of carrier hopping occurs in the tail of Gaussian, exc
Data Loading...
